Previous work from this laboratory has shown that about half the ribosomal RNA (rRNA) genes in Drosophila are interrupted, and that these interrupted genes are inactive, i.e., they are pseudogenes. The molecular basis for the inactivity of these genes is being studied with the aid of a transient expression system using Drosophila cells transfected with rDNA minigenes. Several minigene constructs have been and are being prepared in which the rDNA promoter is placed close to the point of interruption of the rRNA coding sequence by different insertions. Some such constructs initiate transcription at the normal 5' end of rRNA after introduction into cultured cells. The maternal effect developmental gene fs(1)h is being studied in collaboration with Drs. Gans and Forquignon in Gif. This gene is known to lead to homeotic transformations under certain conditions, and provides an example of a maternal effect gene apparently involved in the specification of body plan. A chromosomal walk across the region containing the fs(1)h gene has been carried out. The gene has been located by mapping three mutations in fs(1)h onto the DNA. The mutations include a reciprocal translocation to the third chromosome, a duplication of several kb of wild-type DNA, and the insertion of a transposable element. These three mutations map within a region of about 5kb. A second gene, lethal(1) myospheroid, has been located within the isolated stretch of DNA. Transcription mapping of the relevant regions is proceeding, and evidence for two transcripts from the region has been obtained.